During the combustion process in an IC engine (e.g., a diesel-powered engine), sulfur is concurrently formed with carbon monoxide (CO) and hydrocarbons (HC) as various sulfur oxides (SOx). Typically, 97-99% of the total amount of SOx present in exhaust gas includes sulfur dioxide (SO2) and 1-3% includes sulfur trioxide (SO3). Thus, fuels with higher sulfur content tend to produce higher amounts of SO3. For example, fuel with sulfur content of 1000 ppm may form approximately 1-3 ppm SO3.
Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Conventional exhaust gas aftertreatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines can include a selective catalytic reduction (SCR) catalyst to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3). Generally in such conventional aftertreatment systems, an exhaust reductant, (e.g., a diesel exhaust fluid such as urea) is injected into the aftertreatment system to provide a source of ammonia, and mixed with the exhaust gas to partially reduce the SOx and/or the NOx gases. The reduction byproducts of the exhaust gas are then fluidically communicated to the catalyst included in the SCR aftertreatment system to decompose substantially all of the SOx and NOx gases into relatively harmless byproducts which are expelled out of such conventional SCR aftertreatment systems.
Conventional aftertreatment systems can also include one or more catalysts for pretreatment and/or post treatment of the exhaust gas. For example, some conventional aftertreatment systems for treating diesel exhaust gas can also include a diesel oxidation catalyst, and/or an ammonium oxidation catalyst. Other components can also include a filter. Each of these components is disposed in conventional aftertreatment systems within the length constraints of the aftertreatment system imposed by the dimensions (e.g., length) of the machine generating the exhaust gas. While it is desirable that the residence time of the exhaust gas in the SCR system is maximized, length constraints of the aftertreatment can limit the dimensions of the components of the aftertreatment system.